Method for prospecting for petroleum



United States Patent O 3,305,317 METHOD FOR PROSPECTING FOR PETROLEUM James E. Cooper and Keith A. Kvenvolden, Dallas, Tex., assignors to Mobil Oil Corporation, a corporation of New York No Drawing. Filed Apr. 19, 1963, Ser. No. 274,304 3 Claims. (Cl. 23-230) This invention relates to prospecting and relates more particularly to a geochemical method for prospecting for petroleum.

Petroleum is found in commercially recoverable quantities in various underground formations. The presence of petroleum in a subterranean formation, however, does not ordinarily manifest itself by readily discernible phenomena at the surface of the earth. Accordingly, various prospecting techniques involving investigation of subterranean and surface structure, lithology, chemistry, and microbiology are employed to give indication as to the location of subterranean petroleum reservoirs.

In various of the known prospecting techniques, search has been made in the surface or near surface layers of the earth for components of petroleum, precursors of petroleum, or materials which may have been derived from petroleum or owe their presence to components of petroleum. These techniques are based upon the theory that a component or components of the petroleum migrate from the petroleum reservoir through the adjoining formations to the surface or near surface of the earth. Where petroleum, precursors thereof, or materials derived there-from or which owe their presence to components of petroleum are found in an area in amounts that are anomalous with respect to the amounts found in adjacent areas, the proximity of a petroleum reservoir has been assumed. However, while these methods have given valuable information with respect to the proximity of the reservoir, they have not been able to give any information as to which particular subterranean formation un-- derlying the area would contain a petroleum reservoir.

Other prospecting techniques, in order to overcome the deficiency of the techniques just mentioned, have involved analysis of the earth material contained in a subterranean formation. In these techniques, a well is drilled from the surface of the earth to a formation of interest and samples are taken from the formation and analyzed. Thus, for example, commonly in a well drilling operation, cuttings or cores are removed from a formation and are examined for the presence of petroleum. However, these methods depend entirely upon the detection of indigenous petroleum. Stated otherwise, they merely indicate Whether the well has penetrated a petroleum reservoir. Accordingly, these methods have not been able to give any information as to whether a formation which does not contain indigenous petroleum at the location penetrated by the well might contain petroleum at some other location.

It is an object of this invention to provide a method for prospecting for petroleum.

It is another object of this invention to provide a geochemical prospecting method which gives information concerning the possibility that a subterranean formation may contain a petroleum reservoir even though indigenous petroleum is not present at the sampling point.

It is another object of this invention to provide a method for determining whether petroleum has migrated through a subterranean formation.

These and other objects of the invention will become apparent from the following detailed description.

In accordance with the invention, a sample of water is removed from a subterranean formation and analyzed to determine the ratio of the quantity therein between selected fatty acids, which ratio is indicative of the presence in the subterranean formation of a petroleum reservoir, and thereafter, Where the ratio indicates that the formation contains a petroleum reservoir, carrying out drilling operations in said formation to locate the reservoir in order to produce the petroleum contained in it.

Previously, one of us has ascertained that fatty acids are present in recent sediments, ancient sediments, and in waters from formations containing petroleum. One of us has also ascertained, further, that in these fatty acids, those having odd numbers of carbon atoms are present along with those having even numbers of carbon atoms. The fatty acids containing an even number of carbon atoms predominated in amount over the fatty acids containing an odd number of carbon atoms in each case studied. In this connection, it was recognized that the relative abundance of the fatty acids containing an odd number of carbon atoms increase-d from the recent sediments to the ancient sediments to the formation waters. However, the source of the fatty acids containing the odd number of carbon atoms was not known.

This invention is based upon our discovery that the ratio of fatty acids containing an even number of carbon atoms to those containing an odd number of carbon atoms in formation waters is not a random distribution of these acids. Rather, this distribution is indicative of the past history of the formation with respect to the presence or absence of petroleum where this ratio is not more than 1.6, it can be considered for reasons hereinafter to become more apparent that the formation at some location therein contains petroleum.

It is not intended to limit our invention to the consequences of any theory. However, it may be postulated for purposes of this invention that petroleum has an origin in biologic material and this origin accounts for the observed ratio of the amounts of fatty acids containing even and odd numbers of carbon atoms significant with respect to the presence of petroleum in a formation. More specifically, in biologic material, the fatty acids predominating in amounts contain even numbers of carbon atoms. Thus, it would be considered that, where the ratio of the amount of fatty acids containing an even number of carbon atoms to the amount of those containing an odd number of carbon atoms in a formation is low, biologic material could not have been present as a source of petroleum in the formation. Accordingly, it would be considered that the formation would not contain petroleum. On the other hand, while we have no proof that such is the case, it is believed that, with the passage of geological time, in the conversion of the biologic material to petroleum, a decarboxylation of the fatty acids containing even numbers of carbon atoms occurs with the formation of a normal paraffin and a fatty acid containing one less carbon atom. As a result, the fatty acids containing odd numbers of carbon atoms increase and the ratio of the amounts of fatty acids containing even numbers of carbon atoms to those containing odd numbers of carbon atoms decreases. Thus, while it would appear that a high ratio of amounts of fatty acids containing an even number of carbon atoms to those containing an odd number of carbon atoms would be characteristic of organic material, we have discovered that a low ratio, i.e., not more than 1.6, is characteristic of the presence of petroleum in a formation.

In the practice of the invention, a sample of water is removed from a subterranean formation by any suitable means. Thus, the water may be taken from the formation through a well drilled from the surface of the earth to the formation. Thereafter, the water is analyzed for an amount of fatty acids containing an even number of carbon atoms and an amount of fatty acids containing an odd number of carbon atoms.

Where the ratio of the amounts of fatty acids containing an even number of carbon atoms to the amounts of fatty acids containing an odd number of carbon atoms is not more than 1.6, it can be considered, as stated, that the formation contains a petroleum reservoir. If the ratio is not more than 1.6, drilling activities are carried out within the same formation to locate the petroleum reservoir. Thus, a well can be drilled to the formation at a location which, from the standpoint of the structure of the formation, the reservoir would be present.

While a ratio of the amount of all the fatty acids in the water containing an even number of carbon atoms to the amount of all those containing an odd number of carbon atoms not more than 1.6 is indicative of presence of a petroleum reservoir in the formation, it is preferred to analyze the water from the formation for the amounts of only those fatty acids containing 16, 17, and 18 carbon atoms. As we have found, the fatty acids containing 16, 17, and 18 carbon atoms are those in predominant amounts in formation waters where a petroleum reservoir is present. Further, accurate and dependable analysis of formation waters for the amounts of these fatty acids can be readily made. The ratio of the amounts of the fatty acids indicative of the presence of a petroleum reservoir in a formation can be expressed mathematically by the expression: C +C /2C where C C and C are the amounts of the fatty acids containing 16, 17, and 18 carbon atoms, respectively.

Analyses of the water from the formation for the amounts of the fatty acids may be carried out by any suitable procedure. However, care must be taken to employ a procedure which will give accurate results considering the small amounts of fatty acids ordinarily contained in water from a subterranean formation where a petroleum reservoir is present in the formation. In a preferred procedure for analyzing for the amounts of the fatty acids, the water is first treated with an alkali to saponify the fatty acids, if any, contained within the water. For example, to a 1-liter sample of water, 2 grams of potassium hydroxide can be added. The resulting solution is thereafter concentrated by distillation. Thus, for example, the solution can be concentrated to 200 milliliters by distillation. For this purpose, a 2-foot Vigreux column may be employed. The resulting concentrate is then washed with an organic solvent to remove any unsaponifiable organic constituent of the water. Thus, the concentrate can :be washed wtih three portions, each of 50 milliliters, of carbon tetrachloride. The washed concentrate is thereafter acidified in order to form the fatty acids. Hydrochloric acid may be employed for acidification and sufiicient acid is employed such that the concentrate will be acid to litmus. The concentrate is then extracted with carbon tetrachloride to remove and recover the fatty acids and the extract is concentrated by distillation. A 2-foot Vigreux column can be used for this distillation. The concentrate, while still liquid, is transferred to a vial and the remaining solvent is removed employing a stream of air having a temperature of 40 C.

Following removal of the solvent, the acids are treated to form the methyl esters. To the residue, following removel of the solvent, 3 milliliters of a boron trifiuoridemethanol solution are added. In this solution, the boron trifiuoride is in the amount of 9 grams of boron trifiuoride to 75 milliliters of methanol. The mixture is then heated on a steam bath for a period of about minutes. Thereafter, 1 milliliter of benzene and 2 milliliters of water are added to the solution and the mixture is centrifuged. The benzene layer containing the methyl esters is recovered.

A urea adduct of the methyl esters is then prepared. To the benzene solution, which contains the methyl esters of any fatty acids which were in the formation water, are added 3.5 milliliters of saturated urea in methanol. The

mixture is cooled to almost 0 C. and is permitted to remain at this temperature for a period of approximately 12 hours. The urea adduct of the methyl esters appears in the form of crystals. The crystallized solution is centrifuged, the liquid layer is drawn off, and the crystals are Washed with benzene. One-milliliter portions of benzene are satisfactory for washing. The urea adduct is then decomposed with 1 milliliter of water, 0.15 milliliter of benzene is added, and the resulting mixture is centrifuged again to recover the organic layer.

The organic layer is analyzed by a gas chromatographic method for the amount of the fatty acids contained therein. Any suitable type of chromatographic analysis apparatus may be employed for this purpose. For example, an F and M Model 500 gas chromatograph is satisfactory. This instrument is equipped with a 2-foot column packed with 0.3 percent of silicone grease on 60- 80-mesh glass beads. Helium is the gas employed. For analysis, the initial temperature in the column can be C. and can be increased to 300 C. at the rate of 5.6 C. per minute. The temperature can be held at 300 C. until the esters are eluted completely. The gas chromatograph can be calibrated with standard solutions of methyl esters of fatty acids having 16, 17, and 18 carbon atoms or whichever other fatty acids are analyzed for in the formation water. Following analysis, the amount of the fatty acids conaining 16, 17, and 18 carbon atoms, respectively, will be known. The ratio of the amount of acids having 16 and 18 carbon atoms to twice the amount of acids containing 17 carbon atoms can then be calculated.

As stated, Where the ratio of the amount of fatty acids containing an even number of carbon atoms to those containing an odd number of carbon atoms in the water is not more than 1.6, it can be regarded that petroleum is present at some location in the formation. Petroleum, as is known, can migrate within a formation. The petroleum being lighter than water will tend to migrate upwardly with respect to the water so that the petroleum reservoir will be located at an upper portion of the formation and be underlaid by the water. Such locations within a formation, often termed traps, canbe ascertained by the techniques ordinarily employed by geologists for study of formation structure. A well is then drilled at such location, i.e., at such a trap. Where a reservoir is present at such location, the petroleum is produced from the formation to the well and recovered.

Frequently, a formation from which water is obtained and which contains a ratio of amounts of fatty acids having odd and even numbers of carbon atoms indicative of the presence of a petroleum reservoir may be physically separated from the portion of the formation where the petroleum has finally deposited and formed the reservoir in the formation. For example, faulting of the formation may have occurred. Thus, the portion of the formation from which the water has been taken and the portion of the formation in which the petroleum reservoir is located may be separated by a vertical distance from each other depending upon the degree of faulting. The faulting may have been such that the portion of the formation containing the petroleum reservoir may be at either a higher level or a lower level than the portion of the formation from which the water was taken. Stated generally, drilling would be carried out in any formation which is the same lithologic unit as the formation from which the water was taken and where the ratio in the water of the amount of fatty acids containing an even number of carbon atoms to those containing an odd number of carbon atoms is not greater than 1.6.

The following examples will be illustrative of the invention.

In each of these examples, a sample of water was taken from a subterranean formation. Each of the samples was treated in accordance with the procedure previously described for analysis for fatty acids and the amounts of the acids containing 16, 17, and 18 carbon atoms were Example number: C +C /2C 1 1.20 2 1.05 3 0.84

It will be observed that the value of the ratio for each of these formation Waters was less than 1.6. Drilling in each of these formations demonstrated presence of petroleum in commercial quantities.

Having thus described our invention, it will be understood that such description has been given by way of illustration and example and not by way of limitation, reference for the latter purpose being had to the appended claims.

We claim:

1. A process for prospecting for petroleum in a subterranean formation comprising:

(a) removing a portion of water contained in said formation,

(b) analyzing said water for the amounts therein of fatty acids containing 16, 17, and 18 carbon atoms,

(c) determining the ratio C1 +c13/2C1q Where C C and C are the amounts of the fatty acids containing 16, 17, and 18 carbon atoms, respectively, and

(d) where said ratio is not more than 1.6, indicating the presence of a petroleum reservoir in said formation, carrying out drilling operations in said formation to locate said petroleum reservoir.

2. A process for prospecting for petroleum in a subterranean formation comprising:

(a) drilling a well from the surface of the earth to said formation,

(b) removing a portion of Water contained in said formation through said well,

(c) analyzing said water for the amounts therein of fatty acids containing 16, 17, and 18 carbon atoms,

(d) determining the ratio C +C /2C where C C and C are the amounts of fatty acids containing 16, 17, and 18 carbon atoms, respectively, and

(e) where said ratio is not more than 1.6, indicating the presence of a petroleum reservoir in said formation, carrying out drilling operations in said formation to locate said petroleum reservoir.

3. A process for prospecting for petroleum in a subterranean formation comprising:

(a) drilling a well from the surface of the earth to said formation,

(b) removing a portion of water contained in said formation through said well,

(c) analyzing said water for the amounts therein of fatty acids containing 16, 17, and 18 carbon atoms,

(d) determining the ratio C +C /2C where C C and C are the amounts of fatty acids containing 16, 17, and 18 carbon atoms, respectively, and

(e) where said ratio is not more than 1.6, indicating the presence of a petroleum reservoir in said formation, drilling a well from the surface of the earth to a structural trap for petroleum in said formation to locate said petroleum reservoir.

References Cited by the Examiner UNITED STATES PATENTS 2,406,611 8/1946 Kennedy 23-230 2,500,213 3/1950 Stevens 250-43 2,767,320 10/1956 Coggeshall 250-43.5

MORRIS O. WOLK, Primary Examiner.

Z. PAROCZAY, Assistant Examiner. 

1. A PROCESS FOR PROSPECTING FOR PETROLEUM IN A SUBTERRANEAN FORMATION COMPRISING: (A) REMOVING A PORTION OF WATER CONTAINED IN SAID FORMATION, (B) ANALYZING SAID WATER FOR THE AMOUNTS THEREIN OF FATTY ACIDS CONTAINING 16, 17, AND 18 CARBON ATOMS, (C) DETERMINING THE RATIO C16+C18/2C17 WHERE C16, C17, AND C18 ARE THE AMOUNTS OF THE FATTY ACIDS CONTAINING 16, 17, AND 18 CARBON ATOMS, RESPECTIVELY, AND (D) WHERE SAID RATIO IS NOT MORE THAN 1.6, INDICATING THE PRESENCE OF A PETROLEUM RESERVOIR IN SAID FORMATION, CARRYING OUT DRILLING OPERATION IN SAID FORMATION TO LOCATE SAID PETROLEUM RESERVOIR. 